1. Field of the Invention
The present invention relates to a semiconductor device having a thin film resistor and an alignment mark for determining a trimming position of the thin film resistor.
2. Related Arts
When several semiconductor chips having thin film resistors are provided on a wafer, values of resistance of the thin film resistors are adjusted by laser trimming. In this case, it is necessary to determine precise positions (trimming positions) of the thin film resistors on which the laser trimming is to be performed. Conventionally, as shown in FIG. 9, semiconductor chips 22 are divided by scribe lines 20, and an alignment mark 21 having a cross shape and made of aluminum (Al) is formed on one of intersections of the scribe lines 20 to determine the trimming positions. In more detail, a laser beam is scanned on the alignment mark 21 in X and Y directions, and an absolute position of the alignment mark 21 is recognized based on the contrast of the beam reflected from the alignment mark 21. The trimming positions of the thin film resistors are determined relatively to the absolute position of the alignment mark 21. Then, the laser trimming is performed on the trimming positions using a trimming apparatus.
However, because each width of the scribe lines 20 is very small, there arises a possibility that wiring patterns 22a made of Al and formed on outer circumferential portions of the semiconductor chips 22 are erroneously recognized as the alignment mark 21. As an example, waveforms of the beam reflected from the alignment mark 21 and the wiring patterns 22a are shown in FIG. 10. The waveform corresponding to the alignment mark 21 is not largely different from the waveforms corresponding to the wiring patterns 22a. Therefore, one of the Al wiring patterns 22a may be recognized as the alignment mark 21.
In addition, conventionally, regions other than the scribe lines 20 are covered with a protection layer and a heat treatment is performed on the wafer. Accordingly, the surface of the alignment mark 21 is roughened, resulting in increase in noise of the beam reflected from the alignment mark 21. This also makes difficult to precisely detect the position of the alignment mark 21.